In Northwest Mexico, Insecticide resistance of boll weevil populations has been monitored since 1985 in order to adjust insecticide recommendations. The LD(50) determinations have been conducted at the Yaqui Valley Experimental Station in Cd. Obregon, Son., Mexico with populations collected from the main cotton production areas in this region. LD(50) values of field populations have been contrasted with LD(50) values of boll weevil susceptible colonies in order to obtain resistance ratios for the insecticides under evaluation. In general, some levels of insecticide resistance have been observed to organoclorine cyclodiene group, carbaryl group, methyl parathion group, and pyretroid group.